User Agent and Assistive Technology Support Notes

Calculating size and position can be complex and different browsers can produce different results. This can occur when the CSS styling mixes padding, margins and widths for an object, or when it mixes an offset and plain value, e.g., offsetWidth and width. Some of these behave differently in reaction to zooming. See MSDN: Fix the Box Instead of Thinking Outside It for an explanation of the way that Internet Explorer 6 and later differ from earlier versions of Internet Explorer.

Description

The objective of this technique is to calculate the size and position of elements in a way that will scale appropriately as the text size is scaled.

There are four properties in JavaScript that help determine the size and position of elements:

offsetHeight (the height of the element in pixels)

offsetWidth (the width of the element in pixels)

offsetLeft (the distance of the element from the left of its parent (offsetParent) in pixels)

offsetTop (the distance of the element from the top of its parent (offsetParent) in pixels)

Calculating the height and width using offsetHeight and offsetWidth is straightforward, but when calculating an object's left and top position as absolute values, we need to consider the parent element. The calculatePosition function below iterates through all of an element's parent nodes to give a final value. The function takes two parameters; objElement (the name of the element in question), and the offset property (offsetLeft or offsetTop):

Tests

Procedure

Open a page that is designed to adjust container sizes as text size changes.

Increase the text size up to 200% using the browser's text size adjustment (not the zoom feature).

Examine the text to ensure the text container size is adjusted to accommodate the size of the text.

Ensure that no text is "clipped" or has disappeared as a result of the increase in text size.

Expected Results

Checks #3 and #4 are true.

If this is a sufficient technique for a success criterion, failing this test procedure does not necessarily mean that the success criterion has not been satisfied in some other way, only that this technique has not been successfully implemented and can not be used to claim conformance.